1. Field of the Invention
The present invention is directed to gas process devices, such as mass flow meters, mass flow controllers, gas analyzers, etc., and, more particularly, to a gas process device that is insensitive to changes in gas pressure.
2. Related Art
Many industries, such as the semiconductor and pharmaceutical industries, rely on Mass Flow Controllers (MFCs) to precisely control the amount of gas that is supplied to a particular tool or device. For example, in the semiconductor industry, a number of MFCs are typically used to provide selected amounts of gases to a process device or tool. Typically, each MFC has an inlet that is coupled to a particular supply gas. The outlet of each MFC is typically coupled to a common outlet that is shared among a plurality of MFCs and coupled to the process device or tool. In such systems, gas flow stability remains a chronic problem.
For example, pressure changes in the supply of gas to a MFC may change the mass of gas inside the MFC. As the components within the MFC react to the change in mass, they typically over-compensate for the increase or decrease in pressure, resulting in flow instability. This is particularly noticeable for temporary changes in pressure over a short time period, known as transients, and for low flow rates and heavy gases. For example, a pressure transient as small as 5 pounds per square inch (psi) over 100 milliseconds (msec) may have a dramatic effect on flow.
In addition to pressure transients on the inlet side of an MFC due to, for example, insufficient pressure regulation, flow instability may also result from pressure transients on the outlet side of the MFC. In particular, because several MFCs may be interconnected to a common outlet, changes in the flow provided by one MFC which affect the pressure at the common outlet may also affect the performance of other MFCs coupled to the common outlet. Alternatively, or in addition to the afore-mentioned problem, changes in the pressure of the process device or tool to which the common outlet is connected may affect the performance of one or more MFCs connected thereto. Such pressure transients typically result from transition phases in the gas panel (i.e., the collection of components, supply lines, and conduits connected to the process device or tool) and the interaction between various components in the gas panel, such as between different MFCs, between MFCs and pressure regulators, between MFCs and valves, or between MFCs and the process device or tool itself. During such transition phases, process devices, such as MFCs, may compensate for each other, in effect amplifying each other""s actions. This results in flow and pressure oscillations, commonly referred to as xe2x80x9ccross talk.xe2x80x9d In the semiconductor industry, the inability to accurately control the flow of gas frequently leads to discarding one or more of the first semiconductor wafers being processed. This is referred to as the xe2x80x9cfirst wafer effectxe2x80x9d or xe2x80x9cbad first wafer effect.xe2x80x9d
To avoid problems due to changes in pressure, and, in particular, due to pressure transients, a pressure regulator is frequently added for each fluid process device (e.g., MFC, pressure transducer, etc.) in the gas panel. Although this may mitigate the effect of pressure changes, the use of a pressure regulator for each MFC results in significant cost and maintenance.
According to one embodiment of the present invention, a gas process device is provided having a gas inlet and a gas outlet. The gas process device includes a first restriction, a second restriction, and a gas sensor, each having an inlet and an outlet. The inlet of the first restriction is fluidly coupled to the gas inlet of the gas process device and the inlet of the gas sensor is fluidly coupled to the outlet of the first restriction. The inlet of the second restriction is fluidly coupled to the outlet of the gas sensor and the outlet of the second restriction is fluidly coupled to the gas outlet of the gas process device. According to a further aspect of this embodiment, the second restriction may be constructed and arranged to provide choked flow of a gas.
According to another embodiment of the present invention, a gas process device is provided having a gas inlet and a gas outlet. The gas process device includes a gas sensor fluidly coupled to the gas inlet and the gas outlet, and first and second means for inducing a pressure drop fluidly coupled between the gas inlet and the gas sensor and between the gas sensor and the gas outlet, respectively. According to a further aspect of this embodiment, the gas sensor may include a thermal mass flow meter.
According to another embodiment of the present invention, a pressure insensitive method of measuring a property of a gas in a gas process device having an inlet and an outlet is provided. The method includes acts of receiving a flow of the gas, dropping the pressure of the gas to produce a first reduced pressure gas, providing the first reduced pressure gas to a gas sensor, measuring a property of the gas with the gas sensor, further dropping the pressure of the gas to produce a second reduced pressure gas and providing the second reduced pressure gas to the outlet.
According to a further embodiment of the present invention, a method for ceasing a flow of a gas in a gas process device having a control valve is provided. The method includes acts of closing an inlet valve upstream of the gas process device, maintaining a predetermined drive to the control valve, and closing an outlet valve downstream of the gas process device when a flow rate of the gas drops below a predetermined value so that the flow of gas is ceased in a controlled manner.
According to another embodiment of the present invention, a method for initiating a gas flow in a gas process device having a control valve, a gas sensor, and a restriction downstream of the control valve and the gas sensor is provided. The method includes acts of equalizing a first pressure upstream of the control valve and a second pressure downstream of the control valve and upstream of the restriction with a third pressure downstream of the restriction, opening an inlet valve upstream of the gas process device and an outlet valve downstream of the gas process device, and opening the control valve to a position corresponding to a desired flow rate so that the flow of gas is initiated in a controlled manner.